Accessing volume checkbox of a camera in the Stereoscopy section
You can cycle through the areas of the bpy.context.screen untill you find an area with the type VIEW_3D.

import bpy

areas = bpy.context.screen.areas

for area in areas:
    if area.type == "VIEW_3D":
        area.spaces[0].show_stereo_3d_cameras           = False
        area.spaces[0].show_stereo_3d_convergence_plane = False
        area.spaces[0].show_stereo_3d_volume            = True
        area.spaces[0].stereo_3d_volume_alpha           = 0.5